Grassland that is south of river toured by twins, say? (7)
I believe the answer is:
prairie
'grassland' is the definition.
(I know this)
'that is south of river toured by twins say?' is the wordplay.
'that is' becomes 'i.e.' (short for Latin 'id est', 'that is').
'south of' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation for river).
'toured by' indicates putting letters inside (tour can mean to go around).
'twins say?' becomes 'pair' (a pair of twins).
'r' going inside 'pair' is 'prair'.
'ie' put after 'prair' is 'PRAIRIE'.
(Other definitions for prairie that I've seen before include "Broad rolling tract of land in the US" , "Open grassland of North America" , "Large open area of grassland in the US" , "Grasslands" , "Open grassland in the USA" .)